Method of wdm channel tagging and monitoring, and apparatus

ABSTRACT

Provided is an optical apparatus and method wherein power transfer coefficients arising from SRS are measured at a designated co-location point and the power of dithers, which are impressed on the channels of a multiplexed optical signal propagating through the optical apparatus, is measured at each co-location point. Within the optical apparatus distances between co-location points are short and the power transfer coefficients are effectively constant. Consequently, the power of each channel of the multiplexed optical signal at the co-location points is obtained from the power of the dithers at a respective one of the co-location points and the power transfer coefficients measured at the designated co-location point. In some embodiments, information on the channel power at the co-location points is used to provide instructions for compensating for fluctuations in channel power and/or channel count at an input and/or channel count within the optical apparatus.

RELATED APPLICATION

This application is a divisional of U.S. application Ser. No. 10/067,910filed Feb. 8, 2002 and claims the benefit thereof.

FIELD OF THE INVENTION

The invention relates to the field of optical networks. Morespecifically the invention pertains to wavelength division multiplexedsystems.

BACKGROUND OF THE INVENTION

Performance monitoring to determine channel presence and per-channelpower has been employed using detection of an entire spectrum ofchannels of an optical signal wherein each channel has had impressedupon it a known dither. The dither can be implemented in many ways, suchas amplitude modulation (AM) of a tone or set of tones or a codedivision multiple access (CDMA) modulation scheme, etc.

In an example using AM detection each channel of a multiplexed opticalsignal has impressed upon it a unique AM tone and power of each channelof the optical signal is measured through differentiation of the uniqueAM tones and knowledge of a fixed modulation depth and total opticalpower. This technique has the benefit of being easy to implement andrequires a broadband optical tap and a PIN detector to perform themonitoring function. In addition, one can uniquely identify the sourceof an optical signal by modulating AMs with wave identification (WID)information. The accuracy of this method in estimating per channel poweris limited by effects such as stimulated Raman scattering (SRS) whichlimits its usage to systems with a small number of spans, small channelcounts and low per channel powers.

Modern systems are striving to achieve greater span, higher channelcount and require higher channel power, thus limiting the applicabilityof this approach. Another method is to use an optical spectrum analyzer(OSA) at each monitoring point or a single OSA with an optical switchfor all points that are co-located (along the path of the opticalsignal). This approach provides accurate per-channel power measurementsthat are not affected by SRS. However, in the past, OSAs have not beenequipped with the capability of discerning WID information. In addition,it is usually much more expensive to implement, slower in takingmeasurements and the optical switch can limit the overall reliability ofthe OSA function. Furthermore, an OSA takes up more space than a PINdetector and measurements at a plurality of co-location points using oneor more OSAs causes fiber routing and handling issues.

Finally, SRS can affect system performance. Consequently, a measurementof the SRS present in a system is therefore needed so that it can beused to control the system to minimize the impact of the SRS.

SUMMARY OF THE INVENTION

The invention is an optical apparatus and method of wavelength divisionmultiplexed (WDM) channel tagging and monitoring. The optical apparatusmeasures power transfer coefficients arising from a non-linear processin the transmission medium, such as SRS. These power transfercoefficients are effectively a measurement of the non-linear processesand may be used in the control and optimization of the transmissionsystem.

The optical apparatus also measures inverse power transfer coefficientsat a designated one or more of a plurality of co-location points. Thecumulative power of dithers, which are impressed on channels of amultiplexed optical signal propagating through the optical apparatus,may also be measured at additional co-location points. The inventiontakes advantage of the fact that within the optical apparatus distancesbetween co-location points are short and the inverse power transfercoefficients are effectively independent of co-location point.Consequently, channel power of each channel of the multiplexed opticalsignal at the co-location points is obtained from the cumulative powerof the dithers at a respective one of the co-location points and fromthe inverse power transfer coefficients calculated from effective powermeasurements at the designated co-location point(s). In this way, a fastdither detection of the power of the dithers occurs independently at aplurality of co-location points while slower but accurate powermeasurements for the calculation of the power transfer coefficients needto be performed only at a single one of the co-location points.Consequently, the disclosed apparatus and method maintain a fast andaccurate way of implementing WDM channel tagging and monitoring. Inaddition, since modulation detection is used at the co-location pointswave identification (WID) information may be extracted, at theco-location points, from the channels of the multiplexed optical signal.In some embodiments, information on the channel power at the co-locationpoints is used to provide instructions to basic functional components ofthe optical device for compensating for fluctuations in input channelpower and channel power within the optical apparatus.

In accordance with a first broad aspect of the invention, provided is amethod of monitoring cross-talk in a multiplexed optical signal having aplurality of channels upon at least one of which is impressed a uniquedither. The monitoring is performed at one point while the dithers areimpressed at another point. Channel power is determined for at least onechannel of the multiplexed optical signal. For each channel in which thechannel power has been determined, a fractional power of at least onedither present upon the channel is also determined. Furthermore, thepower transfer coefficients are determined from the fractional power andthe channel power of the channels whose channel power has beendetermined. In some embodiments a power transfer coefficient, β_(ij), ofthe power transfer coefficients may be determined from a channel power,P_(j), of a channel, j, of the channels of the multiplexed opticalsignal and from a fractional power, β_(ij)P_(j), of a dither, i, of thedithers, upon the channel, j. The power transfer coefficient, β_(ij),may then be calculated using β_(ij)=(β_(ij)P_(j))/P_(j). In addition, insome embodiments, a method of controlling output characteristics of themultiplexed optical signal may include the above method and also includeproviding instructions for controlling the power transfer coefficients.

In accordance with another embodiment of the invention, provided is amethod of determining channel power of at least one of a plurality ofchannels of a multiplexed optical signal. One or more unique dithers areeach impressed upon a respective channel of the plurality of channels. Afractional power of each one of the dithers present upon at least one ofthe plurality of channels is determined. Power transfer coefficients arethen determined from the fractional power and from the channel power ofthe channels whose channel power has been determined. The power of thedithers is also determined at at least one other co-location point andthen, for each one of the dithers, respective contributions to thechannel power of at least one of the plurality of channels are summedwherein the respective contributions at the at least one otherco-location point are obtained from the power transfer coefficients andthe power of the dithers.

In accordance with another embodiment of the invention, provided is amethod of WDM channel tagging and monitoring. The method is applied to amultiplexed optical signal having a plurality of channels. One or moreunique dithers are each impressed upon a respective channel of theplurality of channels. The method includes determining, at a designatedone of a plurality of co-location points, inverse power transfercoefficients, β′_(ij), of a matrix [β′]. The method then includesdetermining, at at least one other co-location point, a power, AM_(i),of the unique dithers wherein the power, AM_(i), forms components of avector [AM]. The channel power, P_(j), of at least one of the channelsof the multiplexed optical signal at the other co-location points isthen calculated using [P]=[β′][AM] wherein [P] is a vector withcomponents corresponding to the channel power, P_(j), at the otherco-location points and wherein the components, AM_(i), of the vector[AM] are determined at the other co-location points.

The power, AM_(i), of the unique dithers might also be determined at thedesignated co-location point. Furthermore, the inverse power transfercoefficients, β′_(ji), may be determined by determining the channelpower, P_(j), at the designated co-location point. A fractional power,β_(ij)P_(j), of a dither i of the unique dithers present upon a channelj of the multiplexed optical signal, wherein β_(ij) corresponds to powertransfer coefficients, might also be determined. The power transfercoefficients, β_(ij), may then be calculated from information on thechannel power, P_(j), and the fractional power, β_(ij)P_(j) and usingβ_(ij)=(β_(ij)P_(j))/P_(j). The inverse power transfer coefficients,β′_(ji), might then be calculated through inversion of a matrix [β],which may have as matrix elements the power transfer coefficients,β_(ij), to obtain the inverse matrix [β′]. The sub-scripts of theinverse power transfer coefficients, β′_(ji), may have been reversedwith respect to the power transfer coefficients, β_(ij), for notationalconvenience such that channel powers always use the sub-script j anddither powers always use the sub-script i. The calculated channel power,P_(j), and the inverse power transfer coefficients, β′_(ji), might havecomplex values in which case channel power might be determined by takingan absolute value of the channel power, P_(j).

The method might be applied to a multiplexed optical signal in whichtransfer of dithers from any one of its channels to any other one of itschannels is due to stimulated Raman scattering (SRS). Furthermore, atleast one the channels of the multiplexed optical signal might beimpressed with a plurality of dithers to provide wave identification(WID) information.

The method may include controlling output characteristics of themultiplexed optical signal by perhaps providing instructions to at leastone of a plurality of basic functional components in response tofluctuations in the channel power, P_(j), and/or channel count of theoptical signal at an input and/or the co-location points. Instructionsmight also be provided to at least one of the basic functionalcomponents in response to fluctuations in the power transfercoefficients, β_(ij), of the matrix [β] at one or more of theco-location points.

In accordance with another embodiment of the invention, provided is amethod of wavelength division multiplexed (WDM) channel tagging andmonitoring of a multiplexed optical signal. The multiplexed opticalsignal has a plurality of channels. One or more unique dithers are eachimpressed upon a respective channel of the plurality of channels. Valuesof inverse power transfer coefficients β′_(ji), of a matrix [β′], at adesignated one of a plurality of co-location points are determined.Furthermore, a portion of the multiplexed optical signal from at leastone other co-location point is received and indicators of powers,AM_(i), of the unique dithers are measured wherein the effective powermeasurements, AM_(i), form components of a vector [AM]. The channelpower, P_(j), of at least one of the channels of the multiplexed opticalsignal at the at least one other co-location point is then calculatedusing [P]=[β′][AM] wherein [P] is a vector with components correspondingto the channel power, P_(j), at the at least one other co-location pointand wherein the components, AM_(i), of the vector [AM] are the powers ofthe unique dithers at the at least one other co-location point.

In accordance with another embodiment of the invention, provided is anoptical apparatus which is used to monitor cross-talk in a multiplexedoptical signal at a point in an optical system. The multiplexed opticalsignal has a plurality of channels upon at least one of which has beenimpressed, at another point in the optical system, a unique dither. Theapparatus has an OSA (Optical Spectrum Analyzer) that is used to measurean indicator of channel power of at least one channel of the pluralityof channels. For those channel whose indicator of channel power isdetermined the OSA also measures an indicator of fractional power of atleast one of the dithers present upon the channels. The apparatus alsohas a control circuit used to determine power transfer coefficients fromthe fractional power and the channel power.

In accordance with another embodiment of the invention, provided is anoptical apparatus which is applied to a multiplexed optical signalhaving a plurality of channels. One or more unique dithers are eachimpressed upon a respective channel of the plurality of channels. Theoptical apparatus has an optical spectrum analyzer (OSA) adapted toreceive a portion of the multiplexed optical signal from a designatedco-location point of a plurality of co-location points. The opticalapparatus also has at least one dither detector adapted to receive aportion of the multiplexed optical signal from a respective other one ofthe co-location points. The dither detectors are used to measure apower, AM_(i), of the dithers. The optical apparatus also has a controlcircuit connected to the OSA and to the dither detectors. The controlcircuit is used to calculate inverse power transfer coefficients,β′_(ji), calculated from information on the multiplexed optical signal,at the designated co-location point, obtained from the OSA. The controlcircuit is also used to calculate a channel power, P_(j), of at leastone of the channels of the plurality of channels at the respective otherone of the co-location points based on information associated with thepower, AM_(i), of the dithers at the respective other one of theco-location points and based on the inverse power transfer coefficients,β′_(ji).

The OSA might also be used to measure, from the portion of the opticalsignal received from the designated co-location point, an indicator of afractional power, β_(ij)P_(j), of a dither i of the unique dithers uponon a channel j of the plurality of channels. Furthermore, the OSA mightbe used to measure, from the portion of the optical signal received fromthe designated co-location point, an indicator of the channel power,P_(j), of the plurality of channels.

The control circuit might be further used to provide instructions to atleast one basic functional component for controlling characteristics ofthe multiplexed optical signal at an output in response to fluctuationsin any one or more of the channel power, P_(j), channel count of themultiplexed optical signal and changes in the inverse power transfercoefficients, β′_(ji), at an input and/or the co-location points.

In accordance with another embodiment of the invention, provided is anoptical apparatus which is applied to a multiplexed optical signalhaving a plurality of channels. One or more unique dithers are eachimpressed upon a respective channel of the plurality of channels. Theoptical apparatus has an OSA connected at a designated co-location pointof a plurality of co-location points. The optical apparatus also has atleast one dither detector connected to a respective other one of theco-location points. The dither detectors are used to measure anindicator of a power, AM_(i), of the dithers. The optical apparatus hasa control circuit connected to the OSA and to the at least one ditherdetector. The control circuit is used to calculate inverse powertransfer coefficients, β′_(ji), calculated from information on themultiplexed optical signal, at the designated co-location point,obtained from the OSA. The control circuit also calculates a channelpower, P_(j), of at least one of the channels of the plurality ofchannels at the respective other one of the co-location points based oninformation associated with the power, AM_(i), of the dithers at therespective other one of the co-location points and based on the inversepower transfer coefficients, β′_(ji).

Such an apparatus might have a plurality of basic functional componentsthat are any suitable optical devices.

According to yet another embodiment of the invention, provided is acomputer readable storage medium carrying code. The code is used todetermine, at a designated co-location point of a plurality ofco-location points, values of inverse power transfer coefficients,β′_(ji). The inverse power transfer coefficients, β′_(ji), areassociated with a channel power, P_(j), of channels of a multiplexedoptical signal having a plurality of channels. One or more uniquedithers are each impressed upon a respective channel of the plurality ofchannels. In addition, received from one or more of the co-locationpoints is information associated with a power, AM_(i), of the dithers.The code is then used to calculate channel power, P_(j), of theplurality of channels at the one or more of the co-location points usingthe information associated with the power, AM_(i), of the unique dithersat a respective one of the co-location points and the inverse powertransfer coefficients, β′_(ji). The code may be used to calculate, atperiodic time intervals, new values for the inverse power transfercoefficients, β′_(ji).

The invention enables a well known and widely implemented ditheredchannel tagging and monitoring technique to be applied to WDM systemsthat would normally be hampered by SRS. The result is a simpler, faster,less expensive and more reliable monitoring system.

In addition, the invention enables the measurement of the SRS present inthe system. Since SRS can affect system performance, information on theSRS can be used to control the system in such a way as to minimize theimpact of the SRS.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described withreference to the attached drawings in which:

FIG. 1 is a schematic diagram of an optical line-amplifier implementinga method of wavelength division multiplexed (WDM) channel tagging andmonitoring, in an embodiment of the invention;

FIG. 2A is a schematic diagram of the optical line-amplifier of FIG. 1implementing a controlled response to changing conditions, in anotherembodiment of the invention;

FIG. 2B is a schematic diagram of an optical line-amplifier implementinga controlled response to changing conditions, in another embodiment ofthe invention;

FIG. 3 is a schematic diagram of a PIN detector of FIG. 1;

FIG. 4A is a schematic diagram of an optical spectrum analyzer (OSA) ofthe optical line-amplifier of FIG. 1;

FIG. 4B is a schematic diagram of an optical spectrum analyzer (OSA) ofthe optical line-amplifier of FIG. 1, in another embodiment of theinvention;

FIG. 5 is a schematic diagram of the OSA of the optical line-amplifierof FIG. 1, in another embodiment of the invention;

FIG. 6 is a flow chart of a program used by a control circuit of FIG. 2Ato implement the method of WDM channel tagging and monitoring; and

FIG. 7 is a flow chart of a method used by the program of FIG. 6 todetermine inverse power transfer coefficients, β′_(ji).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In long haul optical networks the accuracy of amplitude modulation (AM)detection or other modulation schemes using wide-band detection of anentire spectrum of channels to determine channel presence andper-channel power is limited by effects of stimulated Raman scattering(SRS). In the modulation schemes at least one channel of a multiplexedoptical signal is impressed with a unique dither. In an example, for amultiplexed optical signal having N channels (carriers) each havingimpressed upon it a unique AM tone (a unique dither) having a fixedmodulation depth and a specific frequency, cross-talk mediated by SRS orother non-linear processes results in transfer of AM tones from onechannel to another. Embodiments of the invention are not limited tohaving all channels of the multiplexed optical signal impressed with aunique dither. In embodiments of the invention at least one of thechannels is impressed with a unique dither. In an optical transmissionsystem the power, AM_(i) (i=1 to N) of the AM tones of the multiplexedoptical signal is given by the following matrix equation:[AM]=[β][P]  (1)where [AM] is a vector with components AM_(i) (i=1 to N) correspondingto the power of the AM tones, [β] is an N×N matrix with power transfercoefficients, β_(ij), (i, j=1 to N) and [P] is a vector with componentsP_(j) (j=1 to N). The components P_(j) correspond to the channel powerof each channel of the multiplexed optical signal. When there is nocross-talk of the AM tones from one channel to another, the matrix [β]has null non-diagonal elements (i.e. β_(ij)=0 when i≠j). Alternatively,for example, in the case of cross-talk mediated by SRS the matrix [β]has non-zero non-diagonal elements and the power transfer coefficients,β_(ij), depend on the distance over which the multiplexed optical signalhas traveled, channel power, channel count and the physical propertiesof the medium that gives rise to SRS.

From equation (1), the channel power of each channel is given by[P]=[β] ⁻¹ [AM]=[β′][AM]  (2)where [β′] is the inverse matrix of [β] , with inverse power transfercoefficients, β′_(ji). Therefore, in optical systems having SRS mediatedcross-talk the inverse power transfer coefficients, β′_(ji), and thepower, AM_(i), of the AM tones are required to calculate the channelpower, P_(j), of each channel of the multiplexed optical signal. In anoptical device, it is sometimes useful to monitor the channel power atdifferent co-location points along the path of the multiplexed opticalsignal as it propagates through different functional components of theoptical device. In principle the inverse power transfer coefficients,β′_(ji), and the power, AM_(i), of the AM tones must be measured at eachco-location point to determine the channel power, P_(j), of each channelof the multiplexed optical signal. However, significant changes in theinverse power transfer coefficients, β′_(ji), occur only over longdistances, of the order of 100 km, as the multiplexed optical signalpropagates through an optical fiber or wave-guide. Within an opticaldevice the distances between co-location points are relatively small andthe inverse power transfer coefficients, β′_(ji), are effectivelyindependent of co-location point. Consequently, while the power, AM_(i),of the AM tones is measured at every co-location point, a measurement ofthe inverse power transfer coefficients, β′_(ji), need only be performedat one of the co-location points to determine the channel power, P_(j),of the channels of the multiplexed optical signal at respective one ofthe co-location points.

Referring to FIG. 1, shown is a schematic diagram of an opticalline-amplifier 5 implementing a wavelength division multiplexed (WDM)channel tagging and monitoring technique, in an embodiment of theinvention. The optical line-amplifier 5 is formed of a series of basicfunctional components comprising input and output erbium-doped fiberamplifiers (EDFAs) 30, 40, respectively, a dynamic gain flattened filterDGFF 50 and a dispersion compensation module (DCM) 60 connected inseries. In other embodiments of the invention, the input and outputEDFAs 30, 40, respectively, are any suitable gain blocks capable ofamplifying an optical signal. A broadband optical tap is insertedbetween any two functional components of the optical line-amplifier 5that are connected together. In particular, broadband optical taps 70,80, 90, 100, 110 are connected at locations between an input opticalfiber 10 and the input EDFA 30 at an input 1, between the input EDFA 30and the DGFF 50, between the DGFF 50 and the DCM 60, between the DCM 60and the output EDFA 40 and between the output EDFA 40 and an outputoptical fiber 20 at an output 2, respectively. Preferably, the broadbandoptical taps 70,80,90,100,110 are asymmetric broadband optical taps andlocations of the broadband optical taps 70,80,90,100,110 are referred toas co-location points. PIN detectors 130,140,150,160,180,190,220 areconnected to a control circuit 370 through inputs430,440,450,460,480,490,420, respectively. In other embodiments of theinvention, the PIN detectors 130,140,150,160,180,190,220 are anysuitable dither detectors capable of detecting dithers. One OSA 230 isconnected to the control circuit 370 through two inputs 431,432. Thecontrol circuit 370 carries out calculations as described herein belowand is preferably implemented as software running on a microprocessor.Alternatively, the software may be impressed as hardware into themicroprocessor.

Connections throughout the optical line-amplifier 5 are achieved througha plurality of optical connectors (only two optical connectors 125,129are shown). The PIN detectors 130,140,150,160 are connected to outputs170 of the broadband optical taps 70,90,100,110, respectively. PINdetectors 180,190 are connected to outputs 200 of the broadband opticaltaps 90,110, respectively.

An optical splitter 210 is connected to output 170 of the broadbandoptical tap 80. The optical splitter 210 has two outputs and ispreferably a 1×2 3-dB optical coupler. A PIN detector 220 is connectedto one of the outputs of the optical splitter 210 and an opticalspectrum analyzer (OSA) 230 is connected to the other output of theoptical splitter 210. In the preferred embodiment of FIG. 1 the OSA 230is connected to the broadband optical tap 80 through the opticalsplitter 210. In other embodiments of the invention, the OSA 230 may beconnected through to any one of the broadband optical taps70,90,100,110.

A multiplexed optical signal M1 carrying N channels propagates along theinput optical fiber 10. Each channel of the multiplexed optical signalM1 is amplitude modulated with a dither having a fixed modulation depthand a unique frequency which is much smaller than the carrier frequency(which happens to be the channel frequency). Each channel of themultiplexed optical signal M1 is therefore amplitude modulated with itsunique AM tone. Preferably, the modulation depth lies in the range 0.02to 0.05. In other embodiments of the invention, each channel of themultiplexed optical signal M1 may be amplitude modulated with one ormore additional dither(s) resulting in a plurality of AM tones impressedupon each channel. In such embodiments, the one or more additionaldither(s) are used to carry wave identification (WID) information of arespective one of the channels.

As described above, in one embodiment of the invention, the dithers areimplemented using an amplitude modulation scheme. Embodiments of theinvention are not limited to an amplitude modulation scheme and othersuitable modulation schemes such as a code division multiple access(CDMA) modulation scheme are used in other embodiments of the invention.In any one of the modulation schemes each channel of the multiplexedoptical signal is modulated with one or more unique dither(s) and AM_(i)generally refers to the power of a dither i.

The multiplexed optical signal M1 propagates into the broadband opticaltap 70 where it is split. A significant portion of the multiplexedoptical signal M1 propagates to the EDFA 30 and a multiplexed opticalsignal S2 that carries a small portion, preferably approximately 2%, ofthe multiplexed optical signal M1 propagates to the PIN detector 130.The multiplexed optical signal M1 is amplified by the EDFA 30 andpropagates into the broadband optical tap 80 where it is split. Asignificant portion of the multiplexed optical signal M1 continues topropagate to the DGFF 50 and a multiplexed optical signal S3 carries asmall portion, preferably approximately 5%, of the multiplexed opticalsignal M1 to the optical splitter 210. The DGFF 50 performs gainequalization over the multiplexed optical signal M1. The multiplexedoptical signal M1 then propagates into the broadband optical tap 90where it is split. A significant portion of the multiplexed opticalsignal M1 propagates to the DCM 60 and a multiplexed optical signal S4carries a small portion, preferably approximately 2%, of the multiplexedoptical signal M1 to the PIN detector 140. The DCM 60 performsdispersion compensation over the multiplexed optical signal M1. Themultiplexed optical signal M1 then propagates into the broadband opticaltap 100 where it is split. A significant portion of the multiplexedoptical signal M1 propagates to the EDFA 40 and a multiplexed opticalsignal S5 carries a small portion, preferably approximately 2%, of themultiplexed optical signal M1 to the PIN detector 150. The EDFA 40amplifies the multiplexed optical signal M1 and the multiplexed opticalsignal M1 then propagates into the broadband optical tap 110 where it issplit. A significant portion of the multiplexed optical signal M1propagates to the output optical fiber 20 and a multiplexed opticalsignal S6 carries a small portion, preferably approximately 2%, of themultiplexed optical signal M1 to the PIN detector 160.

Reflections of the multiplexed optical signal M1 may occur at any one ofthe optical connectors, including optical connectors 125, 129.Reflections of the multiplexed optical signal M1 at the opticalconnector 125 propagate as a reflected multiplexed optical signal R7 tothe broadband optical tap 90. A portion of the reflected multiplexedoptical signal R7 is output at the output 200 of the broadband opticaltap 90 as a multiplexed optical signal S7 that propagates to the PINdetector 180 and a remaining portion of the reflected multiplexedoptical signal R7 propagates to the DGFF 50. Similarly, reflections ofthe multiplexed optical signal M1 at the optical connector 129 propagateas a reflected multiplexed optical signal R8 to the broadband opticaltap 110. A portion of the reflected multiplexed optical signal R8 isoutput at the output 200 of the broadband optical tap 110 as an opticalsignal S8 that propagates to the PIN detector 190 and a remainingportion of the reflected multiplexed optical signal R7 propagates to theoutput EDFA 40.

At the optical splitter 210, the multiplexed optical signal S3 is splitinto a multiplexed optical signal OSA3 that propagates to the OSA 230and split into a multiplexed optical signal PIN3 that propagates to thePIN detector 220. For each channel of the multiplexed optical signalOSA3 the OSA 230 measures an indicator of the channel power, P_(j), (j=1to N where N is the number of channels) of the multiplexed opticalsignal M1. The OSA 230 also measures an indicator of a fractional power,β_(ij)P_(j), of AM tone i present upon channel j of the multiplexedoptical signal M1 (the power, β_(ij)P_(j), is a fraction of the power,AM_(i)). In the preferred embodiment of FIG. 1 the indicators arevoltages and the OSA 230 converts the voltages to powers. This isdiscussed in more detail below with reference to FIGS. 4A, 4B and 5.

Information associated with the powers P_(j) and β_(ij)P_(j) is sent tothe control circuit 370 through inputs 432 and 431, respectively. Thecontrol circuit 370 then calculates the power transfer coefficients,β_(ij), using $\begin{matrix}{\beta_{ij} = {\frac{\beta_{ij}P_{j}}{P_{j}}.}} & (3)\end{matrix}$

Given the matrix elements, β_(ij), the control circuit 370 thencalculates the inverse power transfer coefficients, β′_(ji), of thematrix [β′] by inverting the matrix [β].

The PIN detectors 130,140,150,160,220 are used to measure an indicatorof the power, AM_(i), of AM tones of the multiplexed optical signal M1at the broadband optical taps 70,90,100,110,80 respectively. Similarly,the PIN detectors 180,190 are used to measure the indicator the power,AM_(i), of AM tones of respective ones of the reflected multiplexedoptical signals R7, R8 at the broadband optical taps 90,110,respectively. In the preferred embodiments of FIG. 1 the indicator ofthe power, AM_(i), is a voltage and voltages are converted to powers bythe PIN detectors 130,140,150,160, 180,190,220. An illustrative exampleof measurements of the indicator of the power, AM_(i), is discussedbelow with respect to FIG. 3.

Information associated with the power, AM_(i), of AM tones of themultiplexed optical signal M1 at the broadband optical taps70,80,90,100,110 is sent to the control circuit 370 from a respectiveone of the PIN detectors 130,220,140,150, 160 through a respective oneof the inputs 430,420,440,450,460. Similarly, information on the power,AM_(i), of AM tones of the reflected multiplexed optical signals R7, R8at the broadband optical taps 90,110, respectively is then sent to thecontrol circuit 370 from a respective one of the PIN detectors 180,190through a respective one of the inputs 480,490.

The control circuit 370 receives, at inputs 430,420,440,450,460,information associated with the power, AM_(i), of AM tones of themultiplexed optical signal M1 at co-location points corresponding to thebroadband optical taps 70,80,90,100,110, respectively. The controlcircuit 370 also receives, at inputs 480,490, information on the power,AM_(i), of AM tones of respective reflected multiplexed optical signalsR7, R8 at co-location points corresponding to the broadband optical taps90,110, respectively. Since the co-location points (or equivalently, thebroadband optical taps 70,80,90,100,110) are positioned over a shortspan, the inverse power transfer coefficients, β′_(ji), do not changesignificantly from one co-location point to another. Consequently, theinverse power transfer coefficients, β′_(ji), obtained from measurementsusing the OSA 230 and calculated for a designated co-location pointcorresponding to the broadband optical tap 80 also correspond torespective inverse power transfer coefficients at the co-location pointscorresponding to the broadband optical taps 70,90,100,110. Therefore,given the power, AM_(i), of the AM tones at each co-location point thecontrol circuit 370 calculates the channel power, P_(j), of each channelof the multiplexed optical signal M1 at respective co-location pointsusing equation (2) and from the inverse power transfer coefficients,β′_(ji), obtained from measurements using the OSA 230. In this way, thechannel power of each channel of the multiplexed optical signal M1 ismeasured at a plurality of co-location points along the opticalline-amplifier 5 according measurements of the inverse power transfercoefficients, β′_(ji), at a designated co-location point using a singleOSA. Since measurements by the PIN detectors 130,140,150,160,180,190,220are more efficient (faster and more cost effective) than measurements bythe OSA 230 it is preferable to make use of a single set of measurementsby the OSA 230 to obtain the channel power at the co-location points.

As discussed herein above, PIN detector 220 measures an indicator of thepower, AM_(i), of the AM tones for calculation of the channel power,P_(j), of each channel of the multiplexed optical signal M1 at thebroadband optical tap 80. However, the OSA 230 also measures anindicator of the channel power, P_(j), of each channel of themultiplexed optical signal M1 at the broadband optical tap 80. As such,the PIN detector 220 provides validation measurements by comparingresults obtained from a PIN detector and an OSA at a common co-locationpoint.

Fluctuations in channel power and channel count of the multiplexedoptical signal M1, prior to reaching input 1 of the opticalline-amplifier 5, can occur causing fluctuations in the inverse powertransfer coefficients, β′_(ji). The OSA 230 and control circuit 370therefore perform re-calibration of the inverse power transfercoefficients, β′_(ji), at periodic time intervals. In the preferredembodiment of FIG. 1, the inverse power transfer coefficients, β′_(ji),are measured approximately every hour to compensate for thefluctuations. In other embodiments of the invention, the inverse powertransfer coefficients, β′_(ji), are re-calibrated at time intervals inaccordance with a period of fluctuation of the channel power and channelcount of the multiplexed optical signal M1.

In other embodiments of the invention, an apparatus in which the methodof channel tagging and monitoring is applied need not be an opticalline-amplifier. It may instead be any suitable optical apparatus inwhich monitoring of channel power of channels of a multiplexed opticalsignal propagating through the optical apparatus is required at aplurality of co-location points within the apparatus. In such anembodiment, the inverse power transfer coefficients, β′_(ji), arecalculated from measurements of the channel power, P_(j), and thefractional power, β_(ij)P_(j), at a designated co-location point whichis located along a path defined by a series connection of basicfunctional components of the optical apparatus. In such an embodiment,measurement of the power, AM_(i), of AM tones of a multiplexed opticalsignal and/or reflected portion of it is performed at one or more of theco-location points other than the designated co-location point. Inaddition, in some embodiments, measurement of the power, AM_(i), of AMtones of a multiplexed optical signal and/or reflected portion of it isalso performed at the designated co-location point.

In the preferred embodiment of FIG. 1, the optical line-amplifier 5monitors the channel power of the multiplexed optical signal M1. Thecontrol circuit 370 may also be used to provide a controlled response tofluctuations in the channel power and/or channel count of themultiplexed optical signal M1 at input 1 and/or within the opticalline-amplifier 5. Referring to FIG. 2A, shown is a schematic diagram ofthe optical line-amplifier 5 of FIG. 1 implementing a controlledresponse to changing conditions, in another embodiment of the invention.An optical line-amplifier 6 similar to the optical line-amplifier 5 isshown except that the optical line-amplifier 6 comprises connectionsbetween the control circuit 370 and the input EDFA 30, the output EDFA40, the DGFF 50 and the DCM 60 through outputs 531,540,550,560,respectively. By monitoring the channel power, P_(j), of the multiplexedoptical signal M1 at different co-location points of the opticalline-amplifier 6 and fluctuations therein, the control circuit 370provides instructions to the input and output EDFAs 30,40, respectively,on the required gain and provides instructions to the DGFF 50 and DCM60, for gain equalization and dispersion compensation, respectively. Theinstructions are provided such that output channel power of themultiplexed optical signal M1 at output 2 is independent of fluctuationsin channel power of the multiplexed optical signal M1 at input 1 and/orwithin the optical line-amplifier 6.

In the preferred embodiment of FIG. 2A, the in-line optical amplifier 6is equipped with only one OSA 230 thereby providing only one designatedco-location point. In some embodiments, the DCM 60 contributessignificant amounts of SRS. This is due, for example, to long lengths ofdispersion compensating fiber making up the DCM 60. In such embodiments,in some cases, it is necessary to perform an additional re-calibration.As shown in FIG. 2B, in such embodiments, an additional OSA 231 isinserted at optical tap 100 thereby providing a second designatedco-location point at optical tap 100. Measurements of SRS at the seconddesignated co-location point are used to measure channel power at thesecond designated co-location point and at the co-locations pointcorresponding to optical taps 100 and 110, respectively. Furthermore,measurements of SRS, at optical tap 100, received through an input 451are used by the control circuit 370 to provide instructions, at output531, to the EDFA 30 for controlling amplification and thereby controlinput power to the DCM 60.

In some embodiments of the invention, a number of in-line opticalamplifiers similar to the in-line optical amplifier 6 are connected in aseries connection. In such embodiments, anyone of the in-line opticalamplifiers monitors the power transfer coefficients, β_(ij), of anoptical signal. The in-line optical amplifier that is monitoring thepower transfer coefficients, β_(ij), provides instructions to anotherone of the in-line optical amplifiers from which the optical signal istransmitted to control effects of SRS.

Referring to FIG. 3, shown is a schematic diagram of the PIN detector130 of FIG. 1. The PIN detector 130 is shown as an illustrative exampleof the PIN detectors 130,140,150,160,180,190,220. The PIN detector 130comprises a photodiode detector 340, an electrical amplifier 350, ananalog-to-digital converter (ADC) 355 and a digital signal processor(DSP) 360 connected in series. In other embodiments of the invention,the photodiode detector 340 is any suitable photo-detector capable ofconverting an optical signal into an electrical signal. The DSP 360carries out calculations as described herein below and is preferablyimplemented as software running on a microprocessor. Alternatively, thesoftware may be impressed as hardware into the microprocessor. In otherembodiments, any suitable electrical spectrum analyzer may be used. Themultiplexed optical signal S2 propagates from the broadband optical tap70 to the photodiode detector 340 where it is converted into anelectrical signal that carries information of the multiplexed opticalsignal S2. The electrical signal is amplified through the electricalamplifier 350 and then propagates to the ADC 355 where it is convertedto a digital signal. The digital signal then propagates to the DSP 360where numerical Fourier transforms are applied to the digital signal.Outputs from the numerical Fourier transforms correspond to peak-to-peakvoltages, V_(PINi), associated with the power, AM_(i), of AM tones ofthe electrical signal. The DSP 360 is calibrated to compensate fortemperature dependence, wavelength dependence responsiveness, gain inthe electrical amplifier 350 and effects of tapping only a portion ofthe multiplexed optical signal M1. More specifically, a responsivity,R_(PINi) (where R_(PINi) is in units of Watts/Volt), of the PIN detector130 is measured in a calibration step during manufacture. Thecalibration is done with a light source having a known modulationapplied to it and a known wavelength and optical power. A unitconversion from voltage to power is performed wherein the power, AM_(i),of AM tones is obtained using AM_(i)=R_(PINi)V_(PINi). In the preferredembodiment of FIG. 3, the DSP 360 performs the unit conversion and sendsinformation on the power, AM_(i), of AM tones to the control circuit 370in units of power. In other embodiments of the invention, theresponsivities, R_(PINi), are stored in the control circuit 370 and itis the control circuit 370 that performs the unit conversion afterreceiving information from the DSP 360. In either case, the PIN detector130 measures an indicator of the power, AM_(i), of AM tones.

The PIN detectors 140,150,160,180,190,220 are similar except that a DSPwithin PIN detector 220 is also calibrated for losses due to splittingof the multiplexed optical signal S3 at the optical splitter 210.

In a preferred embodiment of the invention, PIN detectors are used tomeasure an indicator of the power, AM_(i), of AM tones at variousco-location points. In other embodiments of the invention, any AMdetector suitable for detection of the power, AM_(i), of AM tones may beused.

Referring to FIG. 4A, there is shown a schematic diagram of the OSA 230of the in-line amplifier 5 of FIG. 1. The OSA 230 comprises ademultiplexer (DeMUX) 250 and an electrical switch 260 connected througha plurality of parallel paths 270 (only three shown). Preferably, theelectrical switch 260 is a N×1 electrical switch. Within each one of theparallel paths 270 is one of a plurality of photodiode detectors 280 andone of a plurality of electrical amplifiers 290. In other embodiments ofthe invention, the photodiode detectors 280 are any suitablephoto-detectors capable of converting an optical signal into anelectrical signal. The electrical switch 260 is connected to an ADC 300and to an electrical amplifier 310 through an output 265. The electricalamplifier 310 is connected to an ADC 320 and the ADC 320 is connected toa DSP 330. The DSP 330 carries out calculations that are preferablyimplemented as software running on a microprocessor. Alternatively, thesoftware may be impressed as hardware into the microprocessor. In otherembodiments of the invention, the DSP 330 is any suitable electricalspectrum analyzer.

The multiplexed optical signal OSA 3 propagates to the DeMUX 250 whereit is demultiplexed. Each channel of the optical signal OSA 3 propagatesthrough a respective one of the parallel paths 270 where it is convertedinto an electrical signal at a respective one of the photodiodedetectors 280. Each electrical signal associated with respective ones ofthe channels of the multiplexed optical signal OSA 3 is then amplifiedthrough a respective one of the electrical amplifiers 290. Theelectrical signals then propagate to the electrical switch 260 wherethey are switched sequentially in time through output 265. Consequently,at a particular time an electrical signal E_(j) (j=1, 2, . . . , N) isoutput at output 265, a portion E1 _(j) of the electrical signal E_(j)propagates to the ADC 300 where it is converted into a digital signalwhich is sent to the input 432 of the control circuit 370. The digitalsignal received from the ADC 300 provides a voltage, V_(j), to thecontrol circuit 370 at input 432, associated with the channel power,P_(j), of each channel of the multiplexed optical signal M1 at thebroadband optical tap 80. The control circuit 370 calculates the channelpower, P_(j), using P_(j)=R_(j)V_(j) where R_(j) is a responsivity inWatts/Volt through a path j of the parallel paths 270 of the OSA 230.The responsivity, R_(j), is measured during a calibration step duringmanufacturing and stored in the control circuit 370. The calibrationstep is performed using a light source with a known wavelength andoptical power. The responsivity, R_(j), need not be stored in thecontrol circuit 370. In a preferred embodiment of FIG. 4B, theresponsivity, R_(j), is stored in the DSP 330 and the ADC 300 isconnected to the DSP 330. In such an embodiment, the voltage, V_(j), isconverted to the channel power, P_(j), by the DSP 330 and informationassociated with the channel power, P_(j), is sent to the control circuit370 through input 431.

In FIG. 4A, at output 265, another portion E2 _(j) of the electricalsignal E_(j) propagates to the electrical amplifier 310 where it isamplified. The portion E2 _(j) of the electrical signal E_(j) thenpropagates to the ADC 320 where it is converted into a digital signalwhich is then analyzed by the DSP 330 using numerical Fouriertransforms. At one point in time, the output of the numerical Fouriertransforms are peak-to-peak voltage swings, V_(ppij) (i=1, . . . , N)each corresponding to a specific modulation frequency. A peak-to-peakvoltage swing, V_(ppij), is associated with the fractional power,β_(ij)P_(j), of AM tone i of the unique AM tones present upon channel jof the multiplexed optical signal M1. Information on the peak-to-peakvoltage swings, V_(ppij), is sent to the control circuit 370 at input431. The control circuit 370 converts the peak-to-peak voltage swing,V_(ppij), into the fractional power, β_(ij)P_(j), usingβ_(ij)P_(j)=R_(i)V_(ppij) where R_(i) is a wavelength dependentresponsivity in units of Watts/Volt. The wavelength dependentresponsivity, R_(i), is measured in a calibration step, duringmanufacture, using a light source having a known modulation applied toit and a known wavelength and power, and stored in the control circuit370. In other embodiments of the invention the wavelength dependentresponsivity, R_(i), is stored in the DSP 330 and it is the DSP 330 thatconverts the peak-to-peak voltage swing, V_(ppij), into the fractionalpower, β_(ij)P_(j).

Referring to FIG. 5, there is shown a schematic diagram of the OSA 230of the in-line amplifier 5 of FIG. 1, in another embodiment of theinvention. The OSA of FIG. 5 is similar to the OSA of FIG. 4 except thatthe DeMUX 250, the electrical switch 260, the photodiode detectors 280and the electrical amplifiers 290 have been replaced by a serialconnection of a wavelength tunable filter 400, a photodiode detector 380and an electrical amplifier 390. In other embodiments of the invention,the photodiode detector 380 is any suitable photo-detector capable ofconverting an optical signal into an electrical signal. The electricalamplifier 390 is connected to the ADC 300 and to the electricalamplifier 310.

Channels of the optical signal OSA 3 are effectively demultiplexedthrough the wavelength tunable filter 400 by sweeping, at increments oftime, across a range of wavelengths associated with the channels of themultiplexed optical signal OSA 3. The channels of the optical signal OSA3 are therefore output the wavelength tunable filter 400 sequentially intime and each optical signal associated with the channels of themultiplexed optical signal OSA 3 is converted to a respective electricalsignal through the photodiode detector 380. The respective electricalsignal is then amplified by the electrical amplifier 390.

Referring to FIG. 6, shown is a flow chart of a program used by thecontrol circuit 370 of FIG. 2A to implement the method of WDM channeltagging and monitoring. The program is used to calculate the channelpower, P_(j), of each channel of a multiplexed optical signal havingimpressed upon each channel a unique AM tone. At step 900 the inversepower transfer coefficients, β′_(ji), are determined at one of aplurality of co-location points. A method of determining the inversepower transfer coefficients, β′_(ji), which is used by the program, isdescribed herein below with reference to FIG. 7. At step 910,information associated with the power, AM_(i), of the unique AM tones,is received from each one of the co-location points (PIN detectors130,140,150,160,180,190,220). More specifically, the program providesinstructions for receiving such information when channel power isrequired at a co-location point for control and/or monitoring function.At step 920, the channel power, P_(j), is calculated for each one of theco-location points using the measured power, AM_(i), of AM tones at arespective one of the co-location points and the inverse power transfercoefficients, β′_(ji). Step 930 is optional and provides a method ofcontrolling output characteristics of the multiplexed optical signal.Although step 930 is implemented in the control circuit 370 of FIG. 2A,it is not implemented in that of FIG. 1. At step 930, the input andoutput EDFAs 30, 40, respectively, the DGFF 50 and the DCM 60 (the basicfunctional components), through which the multiplexed optical signalpropagates, are adjusted in response to fluctuations in the channelpower and/or channel count of the multiplexed optical signal M1 at input1 and/or the co-location points.

The inverse power transfer coefficients, β′_(ji), may change with timein which case new values need to be determined. Consequently, atperiodic intervals, at step 940, new values for the inverse powertransfer coefficients, β′_(ji), to be used in subsequent calculations ofthe channel power, P_(j), are determined by stepping up to step 900;otherwise the same values for the inverse power transfer coefficients,β′_(ji), are used in the next calculation of the channel power, P_(j),by stepping to step 910 and determining new values for the power,AM_(i), of AM tones.

Referring to FIG. 7, shown is a flow chart of a method used by theprogram of FIG. 6 to determine the inverse power transfer coefficients,β′_(ji). At step 1000, received is the information associated with thechannel power, P_(j), of each channel of the multiplexed optical signalM1 at the designated co-location point. At step 1010, received is theinformation associated with the fractional power, β_(ij)P_(j), of AMtone i of the unique AM tones present upon channel j of the multiplexedoptical signal M1 at the designated co-location point. Moreparticularly, the program provides instructions to an OSA for performingmeasurements to update the matrix [β]. Instructions are carried out attime intervals that are shorter than a time scale associated withchanges in SRS contributions. In embodiments of the invention whereinmore than one unique AM tone per channel have been impressed onto thechannels of the multiplexed optical signal to provide WID information.Any of these AM tones may be used in the calculation of the powertransfer coefficients, β_(ij), and the WID information is extracted froma sequence defined by the AM tones. The power transfer coefficients,β_(ij), are then calculated, at step 1020, from the fractional power,β_(ij)P_(j), and the channel power, P_(j), using equation (3). Thematrix [β] is then inverted at step 1030 resulting in inverse matrix[β′] of which the inverse power transfer coefficients, β′_(ji), are itsmatrix elements. Inversion of the matrix [β] is performed using anysuitable matrix inversion algorithm such as LU decomposition. At step1040, a unit conversion is applied to the inverse power transfercoefficients, β′_(ji), such that the channel power, P_(j), can beproperly calculated, in units of power, using equation (2).

As discussed herein above, SRS causes AM tones to be transferred toother wavelengths. More particularly, AM tones that are transferred tolonger wavelengths are transferred in-phase. Alternatively, AM tonesthat are transferred to shorter wavelengths are transferred 180° out ofphase. Consequently, the power transfer coefficients, β_(ij), can bepositive or negative. More particularly, with β_(ij)≦0 when i>j andβ_(ij)≧0 when i≦j. This is true in cases where there is no chromaticdispersion an in such cases the channel power, P_(j), may be obtainedfrom absolute values of the power transfer coefficients, β_(ij). Inother cases where there is dispersion such a method only works when thedispersion is low. To properly deal with cases in which there isdispersion the power transfer coefficients, β_(ij), of the matrix [β]are treated as complex values, each having a phase, due to SRS, whichmay have any value between 0° and 360°. Since the OSA 230 makes powermeasurements sequentially in time, the power transfer coefficients,β_(ij), also have a phase due to differences in time at whichmeasurements by the OSA 230 are done. The electrical switch 260 allowsmeasurement of power a single channel at a time. For example, a channelj of the multiplexed optical signal M1 has channel power, P_(j), and atone point in time the OSA measures the channel power, P_(j). There at apoint in time the vector [P] as only one non-zero component, P_(j), andfrom equation (1), the power, AM_(i), of AM tone i is then given by$\begin{matrix}{\begin{bmatrix}{AM}_{1} \\{AM}_{2} \\\vdots \\{AM}_{N}\end{bmatrix} = {\begin{bmatrix}\beta_{1,1} & \beta_{1,2} & \cdots & \beta_{1,N} \\\beta_{2,1} & \beta_{2,2} & \quad & \vdots \\\vdots & \quad & ⋰ & \quad \\\beta_{N,1} & \cdots & \quad & \beta_{N,N}\end{bmatrix}\begin{bmatrix}0 \\\vdots \\P_{j} \\0\end{bmatrix}}} & (4)\end{matrix}$which gives $\begin{matrix}{\begin{bmatrix}{AM}_{1} \\{AM}_{2} \\\vdots \\{AM}_{N}\end{bmatrix} = \begin{bmatrix}{\beta_{1,j}P_{j}} \\{\beta_{2,j}P_{j}} \\\vdots \\{\beta_{N,j}P_{j}}\end{bmatrix}} & (5)\end{matrix}$Consequently, the phase due the time at which measurements are done isthe same for each power transfer coefficient, β_(ij), with a column ofthe matrix [β]. The matrix [β] is therefore written as $\begin{matrix}\begin{matrix}{\lbrack\beta\rbrack = \begin{bmatrix}{{\mathbb{e}}^{- {\mathbb{i}\varphi}_{1}}\beta_{1,1}} & {{\mathbb{e}}^{- {\mathbb{i}\varphi}_{2}}\beta_{1,2}} & \cdots & {{\mathbb{e}}^{- {\mathbb{i}\varphi}_{N}}\beta_{1,N}} \\{{\mathbb{e}}^{- {\mathbb{i}\varphi}_{1}}\beta_{2,1}} & {{\mathbb{e}}^{- {\mathbb{i}\varphi}_{2}}\beta_{2,2}} & \quad & \vdots \\\vdots & \quad & ⋰ & \quad \\{{\mathbb{e}}^{- {\mathbb{i}\varphi}_{1}}\beta_{N,1}} & \cdots & \quad & {{\mathbb{e}}^{- {\mathbb{i}\varphi}_{N}}\beta_{N,N}}\end{bmatrix}} \\{= \begin{bmatrix}{z_{1}\beta_{1,1}} & {z_{2}\beta_{1,2}} & \cdots & {z_{N}\beta_{1,N}} \\{z_{1}\beta_{2,1}} & {z_{2}\beta_{2,2}} & \quad & \vdots \\\vdots & \quad & ⋰ & \quad \\{z_{1}\beta_{N,1}} & \cdots & \quad & {z_{N}\beta_{N,N}}\end{bmatrix}}\end{matrix} & (6)\end{matrix}$where z₁=e^(−iΦj) and Φ_(j) corresponds to a phase at a time at whichthe channel power, P_(j), is measured by the OSA 230. The phase Φ_(j) isobtained from complex values of the numerical Fourier transformsperformed by the DSP 330 of the OSA 230.

Using equation (6), equation (1) is re-written as $\begin{matrix}{\begin{bmatrix}{AM}_{1} \\{AM}_{2} \\\vdots \\{AM}_{N}\end{bmatrix} = {\begin{bmatrix}{z_{1}\beta_{1,1}} & {z_{2}\beta_{1,2}} & \cdots & {z_{N}\beta_{1,N}} \\{z_{1}\beta_{2,1}} & {z_{2}\beta_{2,2}} & \quad & \vdots \\\vdots & \quad & ⋰ & \quad \\{z_{1}\beta_{N,1}} & \cdots & \quad & {z_{N}\beta_{N,N}}\end{bmatrix} = \begin{bmatrix}P_{1} \\P_{2} \\\vdots \\P_{N}\end{bmatrix}}} & (7)\end{matrix}$

To obtain the channel powers, P_(j), equation (7) is inverted to give$\begin{matrix}\begin{matrix}{\begin{bmatrix}P_{1} \\P_{2} \\\vdots \\P_{N}\end{bmatrix} = {\begin{bmatrix}{z_{1}\beta_{1,1}} & {z_{2}\beta_{1,2}} & \cdots & {z_{N}\beta_{1,N}} \\{z_{1}\beta_{2,1}} & {z_{2}\beta_{2,2}} & \quad & \vdots \\\vdots & \quad & ⋰ & \quad \\{z_{1}\beta_{N,1}} & \cdots & \quad & {z_{N}\beta_{N,N}}\end{bmatrix}^{- 1}\begin{bmatrix}{AM}_{1} \\{AM}_{2} \\\vdots \\{AM}_{N}\end{bmatrix}}} \\{= {\begin{bmatrix}{\beta_{1,1}^{\prime}/z_{1}} & {\beta_{1,2}^{\prime}/z_{1}} & \cdots & {\beta_{1,N}^{\prime}/z_{1}} \\{\beta_{2,1}^{\prime}/z_{2}} & {\beta_{2,2}^{\prime}/z_{2}} & \quad & \vdots \\\vdots & \quad & ⋰ & \quad \\{\beta_{N,1}^{\prime}/z_{N}} & \cdots & \quad & {\beta_{N,N}^{\prime}/z_{N}}\end{bmatrix}\begin{bmatrix}{AM}_{1} \\{AM}_{2} \\\vdots \\{AM}_{N}\end{bmatrix}}}\end{matrix} & (8)\end{matrix}$

From equation (8), the channel power, P_(j), is given by $\begin{matrix}{P_{j} = {{\sum\limits_{i = 1}^{N}{\frac{\beta_{ji}^{\prime}}{z_{j}}{AM}_{i}}} = {\frac{\sum\limits_{i = 1}^{N}{\beta_{ji}^{\prime}{AM}_{i}}}{z_{j}} = \frac{P_{j}^{\prime}}{z_{j}}}}} & (9)\end{matrix}$and a real part of the channel power, P_(j), is given by $\begin{matrix}{\left. P_{j} \right|_{real} = {{{abs}\left( \frac{P_{j}^{\prime}}{z_{j}} \right)} = {{{abs}\quad\left( P_{j}^{\prime} \right)} = \sqrt{\left( {P_{j}^{\prime}P_{j}^{\prime*}} \right)}}}} & (10)\end{matrix}$where P′_(j)* is a complex conjugate of P′_(j) and abs(z_(j))=1.

Equation (10) shows that in embodiments where the channel power, P_(j),is calculated using complex values of the power transfer coefficients,β_(ij), results are independent of the phase, Φ_(j). As such,calculating the channel power, P_(j), in this way provides a suitablemethod of obtaining the channel power, P_(j). Furthermore, this methodhas the benefit that it is also suitable in cases where there isdispersion.

Numerous modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

1. A method of determining a channel power of a multiplexed opticalsignal having a plurality of channels wherein one or more unique dithersare each impressed upon a respective channel of the plurality ofchannels, the method comprising: determining at a designated co-locationpoint of a plurality of co-location points a channel power of theplurality of channels and a fractional power of each one of the one ormore unique dithers present upon each one of the plurality of channels;determining power transfer coefficients from the fractional power andthe channel power of the plurality of channels; determining, at at leastone other co-location point, the power of the one or more uniquedithers; summing respective contributions to the channel power of atleast one of the plurality of channels, at the at least one otherco-location point; and determining the respective contributions to thechannel power, at the at least one other co-location point, from thepower transfer coefficients and the power of the one or more uniquedithers at the at least one other co-location point.
 2. A method ofmeasuring channel power of a multiplexed optical signal having aplurality of channels wherein one or more unique dithers are eachimpressed upon a respective channel of the plurality of channels, themethod comprising: determining, at a designated co-location point of aplurality of co-location points, inverse power transfer coefficients,β′_(ji), of a matrix [β′]; determining, at at least one otherco-location point, a power, AM_(i), of the one or more unique ditherswherein the power, AM_(i), forms components of a vector [AM]; andcalculating at least one of a channel power, P_(j), of the plurality ofchannels, at the at least one other co-location point using [P]=[β′][AM]wherein [P] is a vector with components corresponding to the channelpower, P_(j), at the at least one other co-location point.
 3. A methodaccording to claim 2 further comprising determining the power, AM_(i),of the one or more unique dithers at the designated co-location point.4. A method according to claim 2 wherein the determining, at adesignated co-location point of a plurality of co-location points,inverse power transfer coefficients, β′_(ji), comprises: determining, atthe designated co-location point, the channel power, P_(j), of theplurality of channels; determining, at the designated co-location point,a fractional power, β_(ij)P_(j), of a dither, i, of the one or moreunique dithers, present upon a channel, j, of the plurality of channels,wherein β_(ij) corresponds to power transfer coefficients; calculatingthe power transfer coefficients, β_(ij), from information on the channelpower, P_(j), at the designated co-location point and the fractionalpower, β_(ij)P_(j), and using β_(ij)=(β_(ij)P_(j))/P_(j); andcalculating the inverse power transfer coefficients, β′_(ji), throughinversion of a matrix [β] comprising the power transfer coefficients,β_(ij), as matrix elements and obtaining the inverse matrix [β′].
 5. Amethod according to claim 2 wherein the channel power, P_(j), and theinverse power transfer coefficients, β′_(ji), have complex values andwherein channel power is determined by taking an absolute value of thechannel power, P_(j).
 6. A method according to claim 2 applied to amultiplexed optical signal in which transfer of dithers from any one ofits channels to any other one of its channels is due to stimulated Ramanscattering (SRS).
 7. A method according to claim 2 applied to amultiplexed optical signal having at least one its channels impressedwith a plurality of dithers to provide wave identification (WID)information.
 8. A method according to claim 2 comprising determining newvalues for the inverse transfer coefficients, β′_(ji), at periodicintervals.
 9. A method of controlling output characteristics of themultiplexed optical signal comprising the method of claim 2 and furthercomprising providing instructions to at least one of a plurality ofbasic functional components in response to fluctuations in at least oneof the channel power, P_(j), and channel count of the optical signal atat least one of an input and the plurality of co-location points.
 10. Amethod of controlling output characteristics of the multiplexed opticalsignal comprising the method of claim 4 and further comprising providinginstructions to at least one of a plurality of basic functionalcomponents in response to fluctuations in the power transfercoefficients, β_(ij), of the matrix [β] at one or more of the pluralityof co-location points.
 11. A method of wavelength division multiplexed(WDM) channel tagging and monitoring of a multiplexed optical signalhaving a plurality of channels wherein one or more unique dithers areeach impressed upon a respective channel of the plurality of channels,the method comprising: determining values of inverse power transfercoefficients β′_(ji), of a matrix [β′], at a designated co-locationpoint of a plurality of co-location points; receiving a portion of themultiplexed optical signal from at least one other co-location point anddetermining a power, AM_(i), of the one or more unique dithers whereinthe power, AM_(i), forms components of a vector [AM]; and calculating achannel power, P_(j), of at least one of the plurality of channels ofthe multiplexed optical signal at the at least one other co-locationpoint using [P]=[β′][AM] wherein [P] is a vector with componentscorresponding to the channel power, P_(j), at the at least one otherco-location point.
 12. An optical apparatus applied to a multiplexedoptical signal having a plurality of channels wherein one or more uniquedithers are each impressed upon a respective channel of the plurality ofchannels, the apparatus comprising: an OSA adapted to receive a portionof the multiplexed optical signal from a designated co-location point ofa plurality of co-location points; at least one dither detector adaptedto receive a portion of the multiplexed optical signal from a respectiveother one of the co-location points, the at least one dither detectorfurther adapted to measure an indicator of a power, AM_(i), of the oneor more unique dithers; a control circuit connected to the OSA and tothe at least one dither detector wherein the control circuit is adaptedto calculate inverse power transfer coefficients, β′_(ji), frominformation on the multiplexed optical signal, at the designatedco-location point, obtained from the OSA and to calculate a channelpower, P_(j), of the plurality of channels for at least one of theplurality of channels at the respective other one of the co-locationpoints based on information associated with the power, AM_(i), of theone or more unique dithers at the respective other one of theco-location points and based on the inverse power transfer coefficients,β′_(ji).
 13. An apparatus according to claim 12 further comprising anadditional dither detector adapted to receive a portion of themultiplexed optical signal from the designated co-location point andadapted to measure an indicator of the power, AM_(i), of the one or moreunique dithers.
 14. An apparatus according to claim 12 wherein the OSAis adapted to measure, from the portion of the optical signal receivedfrom the designated co-location point, an indicator of a fractionalpower, β_(ij)P_(j), of a dither, i, of the one or more unique ditherspresent upon a channel, j, of the plurality of channels.
 15. Anapparatus according to claim 14 wherein the OSA is further adapted tomeasure, from the portion of the optical signal received from thedesignated co-location point, an indicator of the channel power, P_(j),of the plurality of channels.
 16. An apparatus according to claim 15wherein the indicator of the fractional power, β_(ij)P_(j), and theindicator of the channel power, P_(j), are voltages and one of the OSAand the control circuit is adapted convert the voltages into powers. 17.An apparatus according to claim 12 wherein the indicator of the power,AM_(i), is a voltage and one of the at least one dither detector and thecontrol circuit is adapted convert the voltage into a power.
 18. Anapparatus according to claim 12 wherein the OSA comprises: ademultiplexer (DeMUX) adapted to demultiplex the, portion of themultiplexed optical signal received from the designated co-locationpoint; an electrical switch connected to the DeMUX through a pluralityof parallel paths such that each path carries a de-multiplexed opticalsignal associated with a respective channel of the portion of themultiplexed optical signal received from the designated co-locationpoint; and a photo-detector within each one of the parallel paths andadapted to convert a respective one of the de-multiplexed opticalsignals into an electrical signal; wherein the electrical switch isadapted to sequentially output, at a single output, the electricalsignals.
 19. An apparatus according to claim 12 wherein the OSAcomprises a wavelength tunable filter adapted to sequentially filterthrough channels of the portion of the multiplexed optical signalreceived from the designated co-location point.
 20. An apparatusaccording to claim 12 wherein the OSA and the at least one ditherdetector each comprise at least one photo-detector adapted to convert arespective optical signal into an electrical signal.